Conventionally, for measurement of various constituents in body fluid, a method has been adopted in which measurement is conducted by use of specific enzymatic substances capable of reacting with specific constituents in the body fluid. Especially, measurement of blood glucose level is important for monitoring a patient's condition. Therefore, self-monitoring of blood glucose is recommended in which daily variation in blood glucose level is monitored by the patient himself or herself.
In the measurement, it is necessary, for example, for the patient to puncture the skin of the patient's finger or the like with a puncturing device having a puncture needle, to press the surroundings of the punctured part with a finger or the like to squeeze out blood, and to take the squeezed-out blood into a disposable sensor mounted to a measuring device. Therefore, this approach has had a problem as to workability.
To help ease the complicated nature of this measurement effort, a device has been proposed in recent years in which a sensor and a puncture needle are integrated.
For instance, Japanese Patent Laid-open No. 2007-54407 describes two electrodes arranged in a tubular puncture needle in which a cavity exists. The electrodes in the puncture needle are connected to a control substrate through electric wires, and measurement of a current is conducted.
In this method, the blood glucose level can be measured by using the blood sampled into the puncture needle. Therefore, the amount of blood needed for measurement can be reduced. In addition, after the patient's skin is punctured with the puncture needle, the measurement can be performed directly in that condition. This makes it possible to alleviate the burden on the patient.
Japanese Patent Laid-open No. Hei 9-94231 describes that an oxygen fixation working electrode, a counter electrode and a reference electrode are arranged in a capillary tube connected to a minute injection needle. In this method, blood is guided into the capillary tube through the minute injection needle by capillarity, and an oxidation current is measured by the three electrodes in the capillary tube.
On the other hand, a method of manufacturing a tubular body having a cavity therein along its longitudinal direction, such as an injection needle, is proposed in Japanese Patent No. 3943390. In this method, first a thin metal sheet is blanked into a development shape of the desired tubular body. Then, the thin sheet in the development shape of the tubular body is gradually rounded by press working, to mold it into a tubular shape.
In the technique described in Japanese Patent Laid-open No. 2007-54407, however, it is necessary, after separate production of a puncture needle and electrodes, to immobilize the electrodes in a minute space inside the puncture needle. Thus, this technique has problems in regard of immobilization accuracy and productivity.
In addition, for accurate detection of a current generated by an enzymatic reaction, it is desirable to keep the distance between the two electrodes constant. When there is unevenness in the immobilization accuracy of the electrodes as above-mentioned, however, dispersion in the inter-electrode distance is generated, making it impossible to perform a highly accurate constituent detection.
The technique described in Japanese Patent Laid-open No. Hei 9-94231 requires insertion of a working electrode, a counter electrode and a reference electrode which are formed in a small-diameter rod-like shape have into a capillary tube. Since the three electrodes are immobilized in the cavity of the capillary tube, it is difficult to arrange the electrodes with high positional accuracy. Therefore, dispersion in inter-electrode distance is liable to be generated, possibly influencing the constituent detection accuracy.
In addition, two members consisting of the puncture needle and the capillary tube are needed, and a step of connecting them together is also needed, leading to a rise in manufacturing cost.